Maeve, you've just lived up to your name by asking a question that has intoxicated scientists for many centuries! Almost any activity requires the use of skeletal muscles, be it just blinking your eyes or running very fast, but it is the brain that makes them move. Shaking your hand with a friend or waving your hand may seem simple, but with over 650 muscles in your body, how does your brain figure out which muscle to signal, how high to raise your hand or how fast you move it?
Just like how lights and switches in your house are connected by wires, our brain and skeletal muscles are connected by the nervous system. The nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal column, while the PNS extends from the CNS to all the other parts of the body, including muscles and the sense organs. Sensory neurons are nerve cells that receive information like touch or sound from receptors in sense organs (like skin) and transmit them to the brain as electrical signals. Motor neurons are nerve cells that transmit signals from the brain to the muscles to be moved. Both motor and sensory neurons are part of the PNS.
Your brain, motor neurons and sensory neurons need to work together to make your muscles move. For example when someone throws a ball to you, your brain receives sensory information from the sensory neurons in your eyes about the approaching ball and makes calculations about which muscles to use. It then signals the appropriate muscles through the motor neurons to position you in the correct position to catch the ball. Sometimes when there is not enough time for the signal to reach the brain, the spinal cord makes the calculations for you and controls the muscles in a pathway called the reflex arc. Remember how you immediately pulled away your hand after touching a hot plate? With training and practice, you can reduce the time your brain or spinal cord takes to calculate and signal your muscles. This is one of the reasons why athletes are able to react much quicker than normal people.
Although very complex and smart, the nervous system can also be tricked. Let's do a simple activity to demonstrate this, but make sure you try this experiment only under the supervision of an adult. Take three tall glasses and fill one with very warm water, one with ice water the the last with normal temperature water. Now grab the warm-water glass with the palm of one hand and the ice-water glass with the palm of the other hand. After holding the two glasses for about a minute, use both your palms to grab the glass with the normal temperature water. What do you notice? What confusing message did your brain get?